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Lemon, Citrus limon / Rutaceae
Postharvest Atmosphere Management
In many countries worldwide, most of the citrus fruit is harvested and kept in storage rooms at temperatures around 2-5ºC. Nevertheless, lemons and limes cannot be stored at temperatures below 10ºC because they are sensitive to chilling injuries. They are black zones with necrosis in the rind. For that reason, lemons are usually stored at 10-12ºC.

Lemons are harvested when they are pale green, and their colouring finishes when they are stored at 95% of relative humidity. This relative humidity must be kept as high as possible, usually between 85 and 95%, in order to avoid the water losses, that are greater when the environment is dry.

The use of controlled atmospheres provide some benefits to the fruit, although they are not used very often due to their high cost.

Contrary to what people think, the internal quality change of lemons during their storage is quite significant. The juice content increases around 16%, originating mainly from the water stored in the skin. The acids content also increases, and in 4 weeks the colour of the skin changes from pale green to yellow.
Postharvest Problems
The most frequent problems are black and red spots in the skin, due to damages by cold. The brown spots in the skin also appear when there is a breakage of the cells that contain essential oils during the fruit handling. The appearance of green and blue mould is also very common.

- Physiological problems

a) Chilling injuries
They are small holes, red and black spots in the skin. These damages may lead to the fruit rotting.

b) Oil spot
The oil spot takes place when the cells of the skin that contain the oil break during the harvesting or handling in the field. The oil inhibits the lemon‘s correct colouring, producing spots in the skin. A careful handling of the fruit diminishes the problem.

c) Aging of the skin
The fruit that is harvested too late and taken to warehouses, where it is kept for a long time at low temperatures, is subject to excessive drying conditions, causing the skin to darken; at the same time, the skin softens and wrinkles around the base.

- Pathological problems

a) Green mould
Caused by the Penicillium digitatum fungus, which penetrates into the skin, surrounding the fruit. The symptoms begin with the appearance of a watery zone in the surface of the skin, followed by the growth of a fungus without colour; this, afterwards, turns into green.

b) Blue mould
Caused by the Penicillium italicum fungus, which penetrates in the healthy skin and spreads to other nearby lemons. The symptoms are similar to those of the green mould but this mould is blue.
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